Remove the unused git-subtree-check.sh script

Summary:
We are currently not using the subtrees. If we want to do so in the
future it can still be pulled in again, and in the meantime it avoids
some maintenance.

See D3958.

Test Plan:

NOP();

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3960